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<ul>
<li>ARC以后引入了<code>__weak</code>的概念来修饰<code>Objective-C</code>对象,使用这个关键字修饰的对象,对象的引用计数不会+1</li>
<li>这个关键字和<code>__unsafe_unretained</code>有些类似,只是在对象释放的时候<code>__weak</code>会将引用的对象置为<code>nil</code>,而<code>__unsafe_unretained</code>不会(__unsafe_unretained会导致野指针产生)</li>
<li>一般使用<code>__weak</code>进行修饰弱引用,典型的例子就是代理。</li>
</ul>
<blockquote>
<figure class="highlight objectivec"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">@property</span> (<span class="keyword">nonatomic</span>, <span class="keyword">weak</span>, <span class="keyword">nullable</span>) <span class="type">id</span> &lt;<span class="built_in">UICollectionViewDelegate</span>&gt; delegate;</span><br></pre></td></tr></table></figure>
</blockquote>
<hr>
<p>1.2 基础概念二</p>
<ul>
<li><p>一个对象在声明的时候,如果什么修饰符都不写,默认是strong,就会导致引用计数加+1,当出了这个对象声明的<code>作用域</code>,引用计数就会-1,如果引用计数为0,这个对象就会被释放</p>
</li>
<li><p>当一个强引用的对象被Block捕获的时候,引用计数就会增加</p>
</li>
</ul>
<figure class="highlight objc"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br><span class="line">24</span><br><span class="line">25</span><br><span class="line">26</span><br><span class="line">27</span><br><span class="line">28</span><br><span class="line">29</span><br><span class="line">30</span><br><span class="line">31</span><br><span class="line">32</span><br><span class="line">33</span><br><span class="line">34</span><br><span class="line">35</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment">//全局block变量</span></span><br><span class="line"><span class="type">void</span> (^block)(<span class="type">void</span>);</span><br><span class="line"></span><br><span class="line"><span class="comment">//默认在block强引用</span></span><br><span class="line"><span class="type">void</span> blockWithStrong(<span class="type">void</span>)&#123;</span><br><span class="line"></span><br><span class="line">    &#123;</span><br><span class="line">        TestObj *obj = [[TestObj alloc] init];</span><br><span class="line">        <span class="built_in">NSLog</span>(<span class="string">@&quot;TestObj对象地址:%@&quot;</span>,obj);</span><br><span class="line">        <span class="built_in">NSLog</span>(<span class="string">@&quot;before block retainCount:%zd&quot;</span>,[obj arcDebugRetainCount]);</span><br><span class="line"></span><br><span class="line">        <span class="comment">//引用计数并不是简单的+1,而是加2,这是由于block在创建的时候在栈上,而在赋值给全局变量的时候,被拷贝到了堆上</span></span><br><span class="line">        <span class="comment">//Block对对象的强引用,导致如果这个Block一直不释放,那么所强引用的对象也就无法释放</span></span><br><span class="line">        block = ^()&#123;</span><br><span class="line">            <span class="built_in">NSLog</span>(<span class="string">@&quot;TestObj对象地址:%@&quot;</span>,obj);</span><br><span class="line">            <span class="built_in">NSLog</span>(<span class="string">@&quot;obj在堆区的block上存在一引用retainCount:%zd&quot;</span>,[obj arcDebugRetainCount]);</span><br><span class="line">        &#125;;</span><br><span class="line"></span><br><span class="line">        <span class="built_in">NSLog</span>(<span class="string">@&quot;after block retainCount:%zd&quot;</span>,[obj arcDebugRetainCount]);</span><br><span class="line"></span><br><span class="line">        <span class="built_in">NSLog</span>(<span class="string">@&quot;堆%@&quot;</span>,[block <span class="keyword">class</span>]);</span><br><span class="line">        <span class="built_in">NSLog</span>(<span class="string">@&quot;栈%@&quot;</span>,[ ^()&#123;<span class="built_in">NSLog</span>(<span class="string">@&quot;TestObj对象地址:%@&quot;</span>,obj); &#125; <span class="keyword">class</span>]);</span><br><span class="line">    &#125;</span><br><span class="line"></span><br><span class="line">    block();</span><br><span class="line">&#125;</span><br><span class="line"><span class="comment">/*</span></span><br><span class="line"><span class="comment">2021-12-06 11:52:43.266609+0800 weakAndStrong[6599:165118] TestObj对象地址:&lt;TestObj: 0x10713b8c0&gt;</span></span><br><span class="line"><span class="comment">2021-12-06 11:52:43.266935+0800 weakAndStrong[6599:165118] before block retainCount:1</span></span><br><span class="line"><span class="comment">2021-12-06 11:52:43.266971+0800 weakAndStrong[6599:165118] after block retainCount:3</span></span><br><span class="line"><span class="comment">2021-12-06 11:52:43.267021+0800 weakAndStrong[6599:165118] 堆__NSMallocBlock__</span></span><br><span class="line"><span class="comment">2021-12-06 11:52:43.267062+0800 weakAndStrong[6599:165118] 栈__NSStackBlock__</span></span><br><span class="line"><span class="comment">2021-12-06 11:52:43.267092+0800 weakAndStrong[6599:165118] TestObj对象地址:&lt;TestObj: 0x10713b8c0&gt;</span></span><br><span class="line"><span class="comment">2021-12-06 11:52:43.267115+0800 weakAndStrong[6599:165118] obj在堆区的block上存在一引用retainCount:1</span></span><br><span class="line"><span class="comment">*/</span></span><br></pre></td></tr></table></figure>

<ul>
<li>只需要将Block将要强引用的对象,弱引用就可以了</li>
</ul>
<figure class="highlight objc"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br><span class="line">24</span><br><span class="line">25</span><br><span class="line">26</span><br><span class="line">27</span><br><span class="line">28</span><br><span class="line">29</span><br><span class="line">30</span><br><span class="line">31</span><br><span class="line">32</span><br><span class="line">33</span><br><span class="line">34</span><br><span class="line">35</span><br><span class="line">36</span><br><span class="line">37</span><br><span class="line">38</span><br><span class="line">39</span><br><span class="line">40</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment">//指定在block中使用weak</span></span><br><span class="line"><span class="type">void</span> blockWithWeak(<span class="type">void</span>)&#123;</span><br><span class="line"></span><br><span class="line">    &#123;</span><br><span class="line">        &#123;</span><br><span class="line">            TestObj *obj = [[TestObj alloc] init];</span><br><span class="line">            <span class="built_in">NSLog</span>(<span class="string">@&quot;TestObj对象地址:%@&quot;</span>,obj);</span><br><span class="line">            __<span class="keyword">weak</span> TestObj *weakObj = obj;</span><br><span class="line">            <span class="built_in">NSLog</span>(<span class="string">@&quot;before block retainCount:%zd&quot;</span>,[obj arcDebugRetainCount]);</span><br><span class="line"></span><br><span class="line">            <span class="comment">//引用计数并不是简单的+1,而是加2,这是由于block在创建的时候在栈上,而在赋值给全局变量的时候,被拷贝到了堆上</span></span><br><span class="line">            <span class="comment">//Block对对象的强引用,导致如果这个Block一直不释放,那么所强引用的对象也就无法释放</span></span><br><span class="line">            block = ^()&#123;</span><br><span class="line">                <span class="built_in">NSLog</span>(<span class="string">@&quot;TestObj对象地址:%@&quot;</span>,weakObj);</span><br><span class="line">                <span class="built_in">NSLog</span>(<span class="string">@&quot;弱引用obj在堆区的block上不存引用:%zd&quot;</span>,[weakObj arcDebugRetainCount]);</span><br><span class="line">            &#125;;</span><br><span class="line"></span><br><span class="line">            <span class="built_in">NSLog</span>(<span class="string">@&quot;after block retainCount:%zd&quot;</span>,[obj arcDebugRetainCount]);</span><br><span class="line"></span><br><span class="line">            <span class="built_in">NSLog</span>(<span class="string">@&quot;堆%@&quot;</span>,[block <span class="keyword">class</span>]);</span><br><span class="line">            <span class="built_in">NSLog</span>(<span class="string">@&quot;栈%@&quot;</span>,[ ^()&#123;<span class="built_in">NSLog</span>(<span class="string">@&quot;TestObj对象地址:%@&quot;</span>,obj); &#125; <span class="keyword">class</span>]);</span><br><span class="line">        &#125;</span><br><span class="line"></span><br><span class="line">        block();</span><br><span class="line">    &#125;</span><br><span class="line"></span><br><span class="line">    <span class="built_in">NSLog</span>(<span class="string">@&quot;block self retainCount:%zd&quot;</span>,[block arcDebugRetainCount]);</span><br><span class="line"></span><br><span class="line">&#125;</span><br><span class="line"><span class="comment">/*</span></span><br><span class="line"><span class="comment">2021-12-06 12:01:12.861537+0800 weakAndStrong[6722:174247] TestObj对象地址:&lt;TestObj: 0x101106550&gt;</span></span><br><span class="line"><span class="comment">2021-12-06 12:01:12.861601+0800 weakAndStrong[6722:174247] before block retainCount:1</span></span><br><span class="line"><span class="comment">2021-12-06 12:01:12.861648+0800 weakAndStrong[6722:174247] after block retainCount:1</span></span><br><span class="line"><span class="comment">2021-12-06 12:01:12.861707+0800 weakAndStrong[6722:174247] 堆__NSMallocBlock__</span></span><br><span class="line"><span class="comment">2021-12-06 12:01:12.861762+0800 weakAndStrong[6722:174247] 栈__NSStackBlock__</span></span><br><span class="line"><span class="comment">2021-12-06 12:01:12.861802+0800 weakAndStrong[6722:174247] 对象被释放了</span></span><br><span class="line"><span class="comment">2021-12-06 12:01:12.861846+0800 weakAndStrong[6722:174247] TestObj对象地址:(null)</span></span><br><span class="line"><span class="comment">2021-12-06 12:01:12.861889+0800 weakAndStrong[6722:174247] 弱引用obj在堆区的block上不存引用:0</span></span><br><span class="line"><span class="comment">2021-12-06 12:01:12.861927+0800 weakAndStrong[6722:174247] block self retainCount:1</span></span><br><span class="line"><span class="comment">*/</span></span><br></pre></td></tr></table></figure>

<hr>
<h5 id="strong使用"><a href="#strong使用" class="headerlink" title="__strong使用"></a>__strong使用</h5><blockquote>
<ul>
<li>除了在Block外使用<code>__weak</code>对对象进行弱引用,我们偶尔还需要在Block内部对弱引用对象进行一次强引用,这是由于, 仅用__weak所修饰的对象,如果被释放,那么这个对象在Block执行的过程中就会变成nil,这就可能会带来一些问题,比如,数组,字典的插入.</li>
<li>正确的做法是,在Block执行的开始,检验弱引用的对象是否还存在,如果还存在,使用__strong进行强引用,这样,在Block执行的过程中,这个对象就不会被置为nil,而在Block执行完毕后,对象的引用计数就会-1,这样就不会导致对象无法释放</li>
</ul>
</blockquote>
<p>注意：<strong>Block从外界所捕获的对象和在Block内部强使用__strong强引用的对象,差别就在于一个是在定义的时候就会影响对象的引用计数(理由就是上面编译后的代码),一个是在Block运行的时候才强引用对象,执行完毕还是会-1</strong></p>
<figure class="highlight objc"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br><span class="line">24</span><br><span class="line">25</span><br><span class="line">26</span><br><span class="line">27</span><br><span class="line">28</span><br><span class="line">29</span><br><span class="line">30</span><br><span class="line">31</span><br><span class="line">32</span><br><span class="line">33</span><br><span class="line">34</span><br></pre></td><td class="code"><pre><span class="line"><span class="type">void</span> blockWithWeakButStrongInBlock(<span class="type">void</span>)&#123;</span><br><span class="line"></span><br><span class="line">    TestObj *obj = [[TestObj alloc] init];</span><br><span class="line">    __<span class="keyword">weak</span> TestObj *weakObj = obj;</span><br><span class="line">    <span class="built_in">NSLog</span>(<span class="string">@&quot;before block retainCount:%zd&quot;</span>,[obj arcDebugRetainCount]);</span><br><span class="line">    block = ^()&#123;</span><br><span class="line">      </span><br><span class="line">	      <span class="comment">//新增：不允许obj在这个执行过程中释放，就需要加入 __strong</span></span><br><span class="line">        __<span class="keyword">strong</span>  TestObj *strongObj = weakObj;</span><br><span class="line">        <span class="keyword">if</span>(! strongObj) <span class="keyword">return</span>;</span><br><span class="line">        <span class="built_in">NSLog</span>(<span class="string">@&quot;TestObj对象地址:%@&quot;</span>,weakObj);</span><br><span class="line">      	<span class="built_in">NSLog</span>(<span class="string">@&quot;TestObj对象地址:%@&quot;</span>,strongObj);</span><br><span class="line"></span><br><span class="line">        <span class="comment">//一般情况下,只使用__weak就能满足大部分的需求了,只有在多线程处理的时候,需要在Block使用下__strong修饰对象</span></span><br><span class="line">        <span class="built_in">dispatch_async</span>(dispatch_queue_create(DISPATCH_QUEUE_PRIORITY_DEFAULT, <span class="literal">NULL</span>), ^&#123;</span><br><span class="line"></span><br><span class="line">            <span class="keyword">for</span> (<span class="type">int</span> i = <span class="number">0</span>; i &lt; <span class="number">1000</span>; i++) &#123;</span><br><span class="line">                <span class="comment">// 模拟一个耗时的任务</span></span><br><span class="line">            &#125;</span><br><span class="line"></span><br><span class="line">            <span class="built_in">NSLog</span>(<span class="string">@&quot;耗时的任务 结束 TestObj对象地址:%@&quot;</span>,strongObj);</span><br><span class="line">        &#125;);</span><br><span class="line">    &#125;;</span><br><span class="line">    <span class="built_in">NSLog</span>(<span class="string">@&quot;after block retainCount:%zd&quot;</span>,[obj arcDebugRetainCount]);</span><br><span class="line">    block();</span><br><span class="line">&#125;</span><br><span class="line"><span class="comment">/*</span></span><br><span class="line"><span class="comment">2021-12-06 13:57:45.843444+0800 weakAndStrong[7840:257213] before block retainCount:1</span></span><br><span class="line"><span class="comment">2021-12-06 13:57:45.843489+0800 weakAndStrong[7840:257213] after block retainCount:1</span></span><br><span class="line"><span class="comment">2021-12-06 13:57:45.843543+0800 weakAndStrong[7840:257213] TestObj对象地址:&lt;TestObj: 0x101704380&gt;</span></span><br><span class="line"><span class="comment">可以看到在耗时任务执行前对象还是存在的,只是在执行完毕了后,对象被释放了,如果我们使用</span></span><br><span class="line"><span class="comment">2021-12-06 13:57:45.843870+0800 weakAndStrong[7840:257721] 耗时的任务 结束 TestObj对象地址:(null)</span></span><br><span class="line"><span class="comment">2021-12-06 13:57:45.843889+0800 weakAndStrong[7840:257213] 对象被释放了</span></span><br><span class="line"><span class="comment">*/</span></span><br></pre></td></tr></table></figure>

</article><div class="post-copyright"><div class="post-copyright__author"><span class="post-copyright-meta">文章作者: </span><span class="post-copyright-info"><a href="mailto:undefined">Albert Yuan</a></span></div><div class="post-copyright__type"><span class="post-copyright-meta">文章链接: </span><span class="post-copyright-info"><a href="https://yllove.gitee.io/2021/12/06/iOS%E5%BC%80%E5%8F%91%E8%BF%9B%E9%98%B6/weak%E5%92%8C-strong%E5%9C%A8Block%E4%B8%AD%E7%9A%84%E4%BD%BF%E7%94%A8/">https://yllove.gitee.io/2021/12/06/iOS%E5%BC%80%E5%8F%91%E8%BF%9B%E9%98%B6/weak%E5%92%8C-strong%E5%9C%A8Block%E4%B8%AD%E7%9A%84%E4%BD%BF%E7%94%A8/</a></span></div><div class="post-copyright__notice"><span class="post-copyright-meta">版权声明: </span><span class="post-copyright-info">本博客所有文章除特别声明外，均采用 <a href="https://creativecommons.org/licenses/by-nc-sa/4.0/" target="_blank">CC BY-NC-SA 4.0</a> 许可协议。转载请注明来自 <a href="https://yllove.gitee.io" target="_blank">Albert Yuan's Blog</a>！</span></div></div><div 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